The term "friction linings" as is used herein will be understood to mean friction members or elements which can be utilized as, or as part of, brakeshoes, brake plates, clutch plates and the like. Such friction members are generally produced in presses, usually in heated presses. A brakeshoe, for example, can comprise three parts, namely, the friction lining, an intermediate layer which generally serves to promote bonding with the support or backing plate, and the backing plate which forms a carrier for the lining and may be composed of steel.
In the past, such brakeshoes have been made in a single pressing operation. This has been the case even when the brakeshoe consisted only of the backing plate and the friction lining, i.e. did not make use of an intermediate layer.
However, in the fabrication of such brakeshoes, other techniques have also been used. For example, the lining, with or without the intermediate layer, can be pressed to its finished form and fastened to the backing plate only after the pressing operation.
To fabricate a friction lining, the press mold of the press is filled with the friction powder and, in some circumstances also with the intermediate-layer powder and, if desired, a backing plate can be placed on the mold. The pressing operation can then begin and generally comprises alternating pressing and varying cycles, the phases permitting gasses evolved during the pressing of the lining material to be released. The lining material generally comprises thermosetting agents and hardens under the heat and pressure of the press. Once the lining is hardened, the press is opened and the finished friction member removed.
It is customary to provide a plurality of such presses in a press installation. A press installation of this type is often constituted as a turntable machine. A given number of hot presses (usually 6, 12, 18 or 24) are fastened around the periphery of a turntable and the turntable is stepped in a circular path so that each of the hot presses in succession will be disposed in a fixedly located handling station or a plurality of such handling or manipulation stations.
At a first of these stations, the finished lining is removed from the hot press. Then the hot press can be cleaned and the parts which might come into contact with the lining or intermediate layer material can be sprayed with a parting or separating agent reducing addition of material to these parts.
At a second manipulation station, the mold is filled with a friction lining mass and the backing plate is then placed upon the mold.
The whole press then closes and the hardening process begins. During the hardening process the hot press moves on a closed path and returns to the first manipulation station at which the finished brake lining is removed and a new cycle is commenced.
Various ways have been proposed for charging of the press. For example, the press can be charged directly with the pulverulent material. This is disadvantageous because the press is not readily accessible from above and it is difficult to ensure a uniform thickness of the intermediate-layer powder over the friction layer powder.
The term "powder" in this sense is intended to mean any supporting material which can be pressed into a friction lining and thus includes fibrous material and granular or flake materials.
To overcome this disadvantage, it is known to charge the press with only the friction layer powder and to cold press the intermediate-layer powder with elevated pressing forces onto the braking plate. The braking plate with the intermediate layer pressed thereon can then be placed on the mold and the main pressing process undertaken in the manner described.
This approach, however, is not applicable to all materials. For example, the intermediate layer may not be retained on the braking plate. Even when the intermediate layer may be retained on the braking plate it is possible that the pressing operation will not effect a satisfactory bond to the intermediate layer because the density of the layer on the backing plate may be too high.
With such materials, the press mold must first be charged with the friction layer powder and this layer must be levelled in the mold. Then the intermediate-layer powder must be filled into the press and leveled. Only then can the backing plate be applied and the press process begun. All of these manipulations, of course, are time consuming so that the utilization of the press is not optimum.
There are hot presses which have been provided with a plurality of press molds. In these presses a number of friction linings or members can be pressed simultaneously. However, individual control of the pressing force is not possible and this can be a major drawback. These hot presses are usually charged with cold prepressed blanks or preforms to reduce the time required for charging the press. In practice, this process has not found application in the production of asbestos-free linings which are the main friction products currently produced because the preform generally is not found to be sufficiently stable. As a consequence, the problems which arise are similar to those which have been described where the intermediate layer cannot be cold pressed to the backing plate.
The powders which may be used have widely divergent properties and require different compaction ratios. The term "compression ratio" is here used to refer to the ratio between the volume of the finish-pressed lining or member to the volume of the powder constituting same prior to compression. The materials most widely used require compaction or compression ratios of up to 1:10.
For such materials, when the press mold is to be charged directly with the friction and/or intermediate-layer powders, the press mold height must be very high to accommodate this compression ratio. When the press is charged with a cold prepressed blank or preform, however, the height of the press mold naturally can be substantially smaller since it need only accommodate the compression ratio actually preformed by the press. Since the height of the press mold contributes materially to the fabrication cost of the friction member, on the one hand because of the capital cost of the mold and on the other hand because of the compression stroke and press-cycle time required, it is advantageous to minimize the height of the press mold. In addition, with a shorter press mold, energy cost can be reduced since the press mold is generally heated over its entire height.
Cold-prepressed preforms, however, cannot always be used.